Method of making a magnetic field sensor

ABSTRACT

A method of making a magnetic field sensor using in situ solid source graphene and graphene induced anti-ferromagnetic coupling and spin filtering, comprising providing a substrate comprising silicon wafers and thermal oxide, performing DC magnetron sputtering, back-sputtering the substrate, growing amorphous carbon on the substrate, sputtering and growing a first ferromagnetic metal surface on the amorphous carbon, annealing the substrate and the amorphous carbon and the first ferromagnetic metal surface, forming a graphene film on the first ferromagnetic metal surface, wherein the first ferromagnetic metal surface comprises NiFe, sputtering and growing a second ferromagnetic film on the graphene film, and capping the second ferromagnetic film with a platinum layer.

This application claims priority to and the benefits of U.S. PatentApplication No. 62/297,550 filed on Feb. 19, 2016, the entirety of whichis herein incorporated by reference.

BACKGROUND

Magnetic field sensors are ubiquitous, they are found as position,rotation and velocity sensors in vehicles and industrial equipment. Theyare also used for current sensing in high power applications and theadvance in magnetic sensors has enabled the low-cost high-density harddisk industry.

For magnetic sensors to be attractive they need to have a highmagnetoresistance (MR>100%), have a linear response in a small magneticfield (%/Oe), and have low resistance (high signal to noise).

Presently, magnetic sensors have high magnetoresistance (MR>100%), forexample MgO based magnetic tunnel barriers, but these have a highresistance due to the nature of the tunnel barrier. Hall sensors arecheap and have a linear response but a very low sensitivity (%/Oe).Anisotropic magneto resistance sensors have low resistance and thus highsignal to noise but very low magnetoresistance (MR<2-3%).

BRIEF SUMMARY OF THE INVENTION

This disclosure describes and demonstrates a magnetic field sensor basedon two anti-ferromagnetically coupled magnetic layers separated bymultilayer graphene, prepared in a single sputter chamber without avacuum break.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a sputter process. Left is RT sputtering ofamorphous carbon and NiFe. Middle, sample after anneal, multi layergraphene has formed on top of the NiFe film. At this point the sample iseither taken out of the chamber for analysis or left in to proceed tothe second magnetic layer. Right is a full sensor stack.

FIG. 2 is an atomic force microscopy image of Graphene on top of NiFe.

FIG. 3 illustrates a raman map of a 5×5 um area on the left, showing auniform coverage, dark means >5 multilayer of graphene. Middleillustrates layer thickness hystogram of 5 by 5 mm² area, almostexclusively multi layer graphene. Right is a Raman spectrum of a typicalspot showing high quality Graphene.

FIG. 4 illustrates raw signal Mx vs applied field. Magnetic studies byvibrating sample magnetometer show the in-plane magnetization of thefull sample.

FIG. 5 illustrates several views. 5A illustrates a schematic and showsthe external applied field. 5B illustrates the magnetic field. 5C is agraph illustrating normalized resistance vs applied field.

DETAILED DESCRIPTION

This disclosure describes and demonstrates a magnetic field sensor basedon two anti-ferromagnetically coupled magnetic layers separated bymultilayer graphene, prepared in a single sputter chamber without avacuum break.

The key advantages include at least the following: 1) the potential forvery high magnetoresistance values because of the spin filtering natureof multi-layer graphene; 2) the absence of hysteretic behavior, crucialfor sensor application, due to the anti-ferro magnetic coupling inducedby the graphene spacer layer; 3) the low resistance of the multilayer,crucial for high signal to ratio; and 4) very high sensitivity, largemagnetoresistance response in a small magnetic field range.

In this disclosure is described how the use of multilayer graphenesynthesized from a layer of sputtered amorphous carbon inducesanti-ferromagnetic coupling between two ferromagnetic layers (ideal formagnetic sensors) and has low resistance and large magnetoresistanceresponse in a small magnetic field.

Electron transport through multi-layer graphene only supports electronswith a momentum K. Graphene in close contact with ferromagnetic surfacessuch as Ni(111) and Co(0002) have theoretically been shown only to haveminority spin carriers with momentum K. For electrons with momentumdifferent than K (the majority spin), the graphene is effectively aninsulator or tunnel barrier. Multilayer graphene acts as a perfect spinfilter between two magnetic layers in a similar fashion as MgO is a spinfilter in MgO based tunnel barriers with TMR>100%.

The magnetic behavior of two magnetic layers separated by a non-magneticlayer depends on the interlayer exchange coupling. They are eitheranti-ferromagnetically coupled, ferromagnetically coupled, or free. Formagnetic memory applications, it is desired that the coupling be free inorder to arbitrarily switch one of the magnetic layers. This is notideal for magnetic sensors, because this implies hysteretic switchingbehavior.

For a sensor, it is desired that the layers are anti-ferromagneticallycoupled, such that the magnetization of the two layers are anti-parallelwhen no magnetic field is applied and an external magnetic field willovercome the exchange interaction and align the magnetizations parallel.The magnetoresistance response results from the change in resistancebetween the anti-parallel and the parallel magnetic orientation. Aferromagnetic coupling would have the two layers always parallel alignedand lead to zero magneto resistance. Multilayer graphene hastheoretically been predicted to induce anti-ferromagnetic coupling.

Graphene is typically synthesized on a metal by exposure to hightemperature ambient in the presence of a source of carbon. Here, in oneexample or embodiment, we will use graphene from solid source, such thatwe can build the whole sensor stack in a single process run in asputtering chamber.

Example 1

The full stack was prepared in situ using DC magnetron sputtering in anAJA sputtering chamber with base pressure less than 3×10⁻⁸ Torr. Thesputtering guns were in a confocal geometry with ˜15 degree of incidenceangle and ˜15 cm of distance between substrate and the gun.

The substrate, Silicon wafers with a 27.5 nm of thermal oxide, wereultrasonically cleaned 15 minutes each in acetone then alcohol. Afterloading, using DC magnetron sputtering, the substrate was back sputteredfor three minutes at 10 W, 100V in 20 mTorr of Argon. 2.5 nm ofamorphous carbon was grown at a rate of 0.31 nm per minute at 100 W in 3mTorr of Ar. 25 nm of Ni₈₀Fe₂₀ was grown at 0.05 nm/sec at 100 W in 3mTorr Ar.

Example 2

An in situ anneal at 875 C. for 10 minutes was performed in a pressurebelow 2×10⁻⁶ Torr. The sample was then allowed to cool down to roomtemperature at 10 C/min. After the sample was cooled down it was eitherunloaded for analysis or a layer of 12 nm Co₉₀Fe₁₀ was sputter depositedand capped with 7.5 nm of Pt, see FIG. 1.

FIG. 5 shows the 4 point resistance of the un-processed film. Fourprobes were place 1 mm apart and a Current Source was used to supplycurrent to the outer contacts, while the potential difference betweenthe two inner contacts was measured by a Keithley 2000 voltmeter in astandard four-probe electrical measurement. The normalized resistanceversus magnetic field is shown in FIG. 5.

FIG. 5A illustrates a standard four probe resistance measurement on theunprocessed film, probe distance is 1 mm and the applied field isparallel to the current direction. A normalized magneto resistance curveshowing a negative 4% effect, inset shows Magnetoresistance curve of GMRdevice with Fe/Cr/Fe multi layer is shown in FIG. 5C.

Atomic force microscopy of the sample after anneal but before the secondmagnetic layer, see FIG. 1B, shows a smooth film (1 nm RMS), see FIG. 2.Raman studies, FIG. 3, show a uniform covering of multilayer grapheneover a 5 by 5 μm area and the Raman spectra of a typical spot shows a Dover G of 0.18 indicating high quality graphene from a solid sourceamorphous carbon layer was obtained on the top surface of NiFe.

Magnetic studies by vibrating sample magnetometer shown in FIG. 4, showthe in-plane magnetization of the full sample (FIG. 1C). The shape ofthe hysteresis loop is indicative of an anti-ferromagnetic coupledsystem.

Here, the magnetization is reduced and plateaus around zero appliedmagnetic field, where the magnetization of the two layers cancel eachother.

The resistance is lower at zero field and higher at large field. This isthe opposite of conventional giant magneto resistance, which has anegative magneto resistance. This is direct confirmation of the spinfiltering of majority carriers in the multilayer graphene spacer. Theoverall resistance of this sample was 12 Ohm.

There are many major advantages of this new method and device.

Some advantages include but are not limited to: 1) simple in situprocessing using standard sputtering techniques; 2) High MR due to spinfiltering of graphene; 3) High sensitivity and linear regime due toAnti-ferromagnetic coupling; and 4) Low resistance, thin well controlledmulti layer graphene spacer layer.

Some further advantages include but are not limited to: 1) Positivemagneto resistance can be combined with conventional negative magnetoresistance in balancing circuits for even greater sensitivity; 2)Creates a diffusion barrier to prevent inter-diffusion of the two metallayers at high temperature needed for subsequent processing; 3) Providescontrol of the film thickness in discrete 0.3 nm steps down tomonoatomic 0.3 nm thickness that is unachievable with competingmaterials; and 4) Removes the dangling bonds inherent inthree-dimensional crystalline and amorphous materials thereby preventingformation of unwanted compounds at the interface between films.

This new magnetic field sensor using In Situ solid source graphene andgraphene induced anti-ferromagnetic coupling and spin filtering can beused in many applications and are superior to the current state of theart because this new sensor overcomes the issues of the current sensorswhich either have low MR (AMR) high resistance (GMR, MTJs) or lowsensitivity (Hall sensors).

Many modifications and variations of the present invention are possiblein light of the above teachings. It is therefore to be understood thatthe claimed invention may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What we claim is:
 1. A method of making a magnetic field sensor using insitu solid source graphene and graphene induced anti-ferromagneticcoupling and spin filtering, comprising: providing a substratecomprising silicon wafers and thermal oxide; performing DC magnetronsputtering to the substrate comprising silicon wafers and thermal oxide;back-sputtering the substrate; growing amorphous carbon on thesubstrate; sputtering and growing a first ferromagnetic metal surface onthe amorphous carbon; annealing the substrate and the amorphous carbonand the first ferromagnetic metal surface; forming a graphene film onthe first ferromagnetic metal surface, wherein the first ferromagneticmetal surface comprises NiFe; sputtering and growing a secondferromagnetic film on the graphene film; and capping the secondferromagnetic film with a platinum layer.
 2. The method of making amagnetic field sensor using in situ solid source graphene and grapheneinduced anti-ferromagnetic coupling and spin filtering of claim 1wherein the steps of sputtering and annealing were performed in situ ina single chamber.
 3. The method of making a magnetic field sensor usingin situ solid source graphene and graphene induced anti-ferromagneticcoupling and spin filtering of claim 2 wherein the thermal oxide is 27.5nm of thermal oxide and the graphene film is multilayer.
 4. The methodof making a magnetic field sensor using in situ solid source grapheneand graphene induced anti-ferromagnetic coupling and spin filtering ofclaim 3 wherein the step of back-sputtering the substrate is performedin situ using DC magnetron sputtering for 3 minutes at 10 W, 100V in 20mTorr of Argon.
 5. The method of making a magnetic field sensor using insitu solid source graphene and graphene induced anti-ferromagneticcoupling and spin filtering of claim 4 wherein the step amorphous carbonwas grown in situ at a rate of 0.31 nm per minute at 100 W in 3 mTorr ofAr.
 6. The method of making a magnetic field sensor using in situ solidsource graphene and graphene induced anti-ferromagnetic coupling andspin filtering of claim 5 wherein the first ferromagnetic metal surfaceis Ni₈₀Fe₂₀ and the step of growing the first ferromagnetic metalsurface was in situ at a rate of 0.05 nm/sec at 100 W in 3 mTorr Ar. 7.The method of making a magnetic field sensor using in situ solid sourcegraphene and graphene induced anti-ferromagnetic coupling and spinfiltering of claim 2 wherein the step of annealing the substrate and theamorphous carbon and the first ferromagnetic metal surface was performedin situ at 875° C. for 10 minutes in a pressure below 2×10⁻⁶ Torr. 8.The method of making a magnetic field sensor using in situ solid sourcegraphene and graphene induced anti-ferromagnetic coupling and spinfiltering of claim 7 further comprising a step of cooling the substrateand the amorphous carbon and the first ferromagnetic metal surface toroom temperature after the step of annealing and prior to the step ofsputtering and growing the second ferromagnetic film on the graphenefilm and wherein the second ferromagnetic film is Co₉₀Fe₁₀.
 9. Themethod of making a magnetic field sensor using in situ solid sourcegraphene and graphene induced anti-ferromagnetic coupling and spinfiltering of claim 7 wherein the platinum layer is 7.5 nm.